1. Field
The present application relates to metrology for semiconductor applications, and in particular to an optical metrology method and system for reducing measurement inaccuracies.
2. Description of Related Art
As integrated circuits (IC) evolve towards smaller geometries of the IC features and faster response times, new challenges are encountered in the manufacturing process. In particular, accurate measurements of the smaller feature sizes are becoming increasingly more difficult. Knowledge of the dimensions of gratings and/or periodic structures, however, is essential to determine if the dimensions of the IC features are within acceptable ranges and if, for example, a particular fabrication process causes sidewalls of the features to be tapered, vertical, T-topped, undercut, have footings, and the like, which can affect final device performance.
Optical metrology has emerged as an effective tool for measuring IC features of small sizes. Optical metrology uses optical signals that are typically non-destructive to the semiconductor materials and small features that are being measured. Further, optical metrology systems can be used for determination of thickness and topographic information, which include CD measurements, as well as optical properties (e.g., refractive index and extinction coefficient n&k) of semiconductor structures.
In one optical metrology system, scatterometry is used to reconstruct a diffraction grating profile from its optical diffraction responses at a fixed incident angle and multiple wavelengths. A library-based methodology for profile extraction is provided, where libraries comprising of simulated optical metrology signals are created for given grating profiles. Alternatively, the method can be carried out for a variety of control parameters beside wavelengths. For example, a fixed wavelength and multiple incident angles, or a hybrid of different angles and wavelengths can be used. An additional example of control parameters besides incident angle and wavelength is illumination polarization state, which can be used alone or in any combination with the two previous cited parameters.
The optical metrology hardware typically used in conjunction with such scatterometry measurements includes, for example, a light source, a detection scheme, and a complex assortment of optical and mechanical components. These various hardware pieces involve parameters that are, with respect to their design values, difficult to set equal from tool-to-tool. Furthermore, with time, and the effect of outside factors, for example, vibrations, pressure, humidity, part replacements, and the like, the parameters within a single tool can vary as well. In addition to the variability of metrology hardware-related parameters, material based parameters such as the optical characteristics n and k, may vary from sample batch to batch, i.e., in different semiconductor wafer batches, or across a single batch of material, i.e., wafer-to-wafer or within-wafer.
Variation in system parameters, i.e., metrology hardware and material parameters, can lead to inaccuracies in CD values, and any other parameters used to describe the structure under inspection, retrieved from optical metrology measurements. In particular, the library diffraction signals are calculated according to certain predefined parameters, often based on a particular set of hardware specifications under expected or average material and operating conditions. If the actual pieces of hardware and material used in the measurement of a sample's diffraction signal differ from those specifications and characteristics used in the library calculations, inaccuracies may occur when attempting to find a best-fit or match of the measured diffraction signal with the calculated library diffraction signals. Thus, because of such considerations, different instruments and materials typically require different libraries corresponding to the instrument and material specifications and characteristics.